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Other galaxies

Other galaxies. Telescopic images of the night sky reveal many other galaxies What do they look like? are they all like the Milky Way? Where are they? spread randomly through space, or grouped? What can we learn about the Universe?. A long time ago in a galaxy far, far away.

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Other galaxies

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  1. Other galaxies • Telescopic images of the night sky reveal many other galaxies • What do they look like? • are they all like the Milky Way? • Where are they? • spread randomly through space, or grouped? • What can we learn about the Universe? Our Evolving Universe

  2. A long time ago in a galaxy far, far away... • How do we know these fuzzy blobs are distant galaxies? • some types of star (especially variable stars) have well-known intrinsic brightness • by measuring how bright they appear to be we can infer their distance • “standard candles” Our Evolving Universe

  3. Types of galaxies • Galaxies seem to come in two basic types • smooth, featureless elliptical galaxies • circular or elongated • made of old, reddish stars • spiral galaxies like theMilky Way • some with round bulges,some with bars Our Evolving Universe

  4. Hubble’s tuning fork no spiral arms Sa Sb Sc lots of young stars old stars; no recent star formation Irregular E0 ••• E6 S0 SB0 looser spiral arms —> smaller bulge —> amorphous or disrupted more elongated —> old stars SBaSBbSBc dwarf elliptical (dE/dSph) old stars in bulge; younger in disc; youngest in spiral arms Our Evolving Universe

  5. Where do we fit in? • The Milky Way is clearly not an elliptical galaxy • it has a disc, and contains young stars • It has spiral arms • so, not S0 • size of bulge and arm pattern suggest Sbc • between Sb and Sc • There is evidence for a small bar • SBbc, or SABbc • SAB means intermediate between barred and unbarred Our Evolving Universe

  6. The Local Group • The Milky Way is not alone: it is part of a small group containing • M31 (the Andromeda galaxy) • a large Sb spiral, bigger than us • M33 • a small Sc spiral • the Large Magellanic Cloud • an irregular satellite of the Milky Way • at least 30 dwarf irregular and dwarfelliptical galaxies • but no large ellipticalgalaxies Our Evolving Universe

  7. Galaxy groups and clusters • The Local Group is small: some rich clusters contain thousands of large galaxies • elliptical and S0/SB0 galaxies are much more common in rich clusters • spiral and irregular galaxies are much more common in small groups and the outskirts of clusters Our Evolving Universe

  8. Elliptical galaxies contain old stars have little net rotation star orbits are randomly directed, as in our halo have little internal structure are much more common in galaxy-rich environments include the most massive galaxies (but also some with very low mass) Spiral galaxies show recent star formation (in disc) have rotating discs stars all orbit in same direction have complex internal structure are more common in low-density environments have a smaller range of masses Galaxy properties Lenticular (S0/SB0) galaxies are like spiral galaxies with no gas Irregular galaxies are mostly like spirals too small to become organised Our Evolving Universe

  9. Galaxy problems • What makes some galaxies elliptical and others spiral? • their mass? • their age? • their rotation? • their history? • How do spiral galaxies avoid “windingup” their spiral arms? • How does the evolution of galaxiesrelate to the presence of centralsupermassive black holes? • the Milky Way’s is, if anything, lessmassive than most! …next lecture! Our Evolving Universe

  10. Galaxies and cosmology • Almost all galaxies are moving away from us • and the greater their distance, the faster they recede (Hubble’s law) • Clusters of galaxies group to form huge superclusters, separated by vast voids • how does this large scale structure develop? • What is the dark matter? …next section! Our Evolving Universe

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